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Abstract:

According to one embodiment, a magnetic recording medium includes: a data
area on which a plurality of first magnetic dots are arranged at
predetermined positions to record information; a servo area on which a
plurality of second magnetic dots for specifying the positions of said
plurality of first magnetic dots are arranged at predetermined positions;
and servo frames configured so that a frequency of said servo frames is 2
N per circumference of said medium having a radius, that said servo
frames are radially discontinuous, and that said servo frame and a
space-area, on which no servo frames exist, are alternately radially
arranged at a cycle W.

Claims:

1. A magnetic recording medium comprising: a data area on which a
plurality of first magnetic dots are arranged at predetermined positions
to record information; a servo area on which a plurality of second
magnetic dots for specifying the positions of said plurality of first
magnetic dots are arranged at predetermined positions; and servo frames
configured so that a frequency of said servo frames is 2 N per
circumference of said medium having a radius, that said servo frames are
radially discontinuous, and that said servo frame and a space-area, on
which no servo frames exist, are alternately radially arranged at a cycle
W.

2. The magnetic recording medium according to claim 1, wherein the number
of said servo frames arranged to touch a circumference of said medium
having a radius r is N per circumference or 2 N per circumference.

3. The magnetic recording medium according to claim 2, wherein when the
number of said plurality of servo frames arranged to touch the
circumference of said medium having a radius r1 is 2 N per circumference,
a radial overlapping-width or1 between adjacent ones of said plurality of
servo frames arranged at a frequency of 2 N per circumference to touch
the circumference of said medium having the radius r1 is set to be equal
to or larger than a read/write offset amount ΔMRr1 of a magnetic
head, which corresponds to the radius r1.

4. The magnetic recording medium according to claim 3, wherein a radius
length L of each of said plurality of servo frames corresponding to a
radius is set such that L=W/2+or1<x_sdomain, based on comparison with
a mark length x_sdomain at which a magnetic film of a servo pattern
configuring each of said plurality of servo frames maintains single
magnetic-domains without generating magnetic walls, the cycle W, and the
overlapping-width or1.

5. A magnetic recording apparatus comprising: a magnetic recording medium
comprising: a data area on which a plurality of first magnetic dots are
arranged at predetermined positions to record information; a servo area
on which a plurality of second magnetic dots for specifying the positions
of said plurality of first magnetic dots are arranged at predetermined
positions; and servo frames configured so that a frequency of said servo
frames is 2 N per circumference of said medium having a radius, that said
servo frames are radially discontinuous, and that said servo frame and a
space-area, on which no servo frames exist, are alternately radially
arranged at a cycle W; and a magnetic head comprising a device configured
to face the surface of said magnetic recording medium to record or
reproduce magnetic information on or from said magnetic recording medium.

Description:

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] The application is based upon and claims the benefit of priority
from Japanese Patent Application No. 2011-120940 filed on May 30, 2011,
the entire contents of which are incorporated herein by reference.

BACKGROUND

[0002] 1. Field

[0003] Embodiments of the present invention relate to a magnetic recording
medium, a magnetic recording apparatus, and a servo control method, which
use a bit-patterned medium.

[0004] 2. Description of the Related Art

[0005] Technology called a "pre-servo" method of forming servo patterns on
a bit-patterned medium simultaneously with the formation of bit patterns
thereon, based on the intensity of magnetization thereof and the volumes
of magnetic materials provided thereon, and then performing servo-writing
by the single-directional magnetization of the entire surface of the
medium is a major candidate for technology applied to the process of
forming a servo pattern. Hitherto, servo patterns configuring servo
frames provided on the medium, e.g., patterns called servo preambles and
servo addresses provided thereon extend continuously from the outer edge
to the inner edge of the medium. If a magnetic film (e.g.,
CoCr--SiO2-based granular film) to be subjected to conventional vertical
magnetic recording is used, the magnetization state thereof is stable
after the servo-writing. However, if a magnetic film (e.g., FePt or CoPt
based single-magnetic-domain film) specialized to a bit-patterned medium
is used, unintended magnetic walls are formed on servo patterns due to
external energy such as thermal disturbance. That is, although all servo
patterns formed in servo frames should be magnetized in a single
direction (e.g., N-direction), the magnetization direction of the servo
frames is naturally changed to the opposite direction (e.g., S-direction)
due to the occurrence of magnetic walls. This causes noise and
address-decoding-error. Effective countermeasures against this phenomenon
are to reduce the areas of the patterns in the servo frames and to
segmentalize each of the patterns. However, from the viewpoint of servo
control, to detect a radial position continuously, it is necessary that
information recorded on each servo frame is normally reproduced at any
radial position.

[0006] Accordingly, it is devised that the patterns are provided at
positions which are radially discrete and circumferentially different
from one another. However, no means is known, which uses the
configuration of the entire servo patterns and implements better
reproduction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] A general configuration that implements the various features of
embodiments will be described with reference to the drawings. The
drawings and the associated descriptions are provided to illustrate
embodiments and not to limit the scope of the embodiments.

[0008] FIG. 1 is a diagram illustrating the arrangement of servo frames on
a medium according to an embodiment of the invention;

[0009] FIG. 2 is an enlarged diagram illustrating the vicinity of servo
frames 2k and 2k+1 at a radius r1 in FIG. 1;

[0010]FIG. 3 is a further enlarged diagram illustrating the vicinity of
the servo frames at the radius r1 and the relation between distances or1
and ΔMRr1 in FIG. 2;

[0011]FIG. 4 is a graph illustrating the occurrence probability of a
magnetic wall versus a mark x length measured at a servo pattern in the
case of using a certain magnetic film;

[0012] FIG. 5 is a diagram illustrating the relation between the orbit of
a magnetic-head used in an embodiment at a seek-operation and a servo
detection timing;

[0013] FIG. 6 is a schematic diagram illustrating the configuration of an
example of a magnetic recording apparatus having a magnetic recording
medium according to the embodiment of the invention;

[0014]FIG. 7 is a block diagram illustrating a servo signal demodulation
circuit in the magnetic disk apparatus according to the embodiment; and

[0015] FIG. 8 is a chart illustrating operation timings of the servo
signal demodulation circuit in the magnetic disk apparatus according to
the embodiment.

DETAILED DESCRIPTION

[0016] Hereinafter, an embodiment of the invention is described with
reference to FIGS. 1 to 8.

[0017] First, FIG. 6 is a schematic diagram illustrating the configuration
of a magnetic recording (reproducing) apparatus having a magnetic
recording medium (to be described below) according to an embodiment of
the invention. The magnetic recording apparatus illustrated in FIG. 6 has
a disk-like magnetic recording medium (magnetic disk medium) 1
(hereinafter, a magnetic recording apparatus having a magnetic disk
medium is referred to as a magnetic disk apparatus).

[0018] The magnetic disk apparatus includes a disk enclosure 101 and a
circuit board 120.

[0019] The disk enclosure 101 is a container that hermetically encloses
the magnetic disk medium 1, a spindle motor (SPM) 102, a magnetic head
103, an actuator 105, a head gimbal assembly 108, a carriage arm 106, a
shaft 110, a head amplifier 107, and the like mounted therein. The
actuator 105 includes a voice coil motor (VCM (not illustrated)). The
magnetic disk medium 1 is mounted on the SPM 102. The magnetic head 103
includes at least one of a recording (write) device (not illustrated)
that records magnetic information on the magnetic disk medium 1, and a
reproducing (read) device (not illustrated) that reads the magnetic
information recorded on the magnetic disk medium 1 as electric signals.
The recording device includes, e.g., a write coil, a main magnetic pole
layer, and an auxiliary pole layer. The write coil has a function of
generating a magnetic flux. The main magnetic pole layer has a function
of containing the magnetic flux generated by the write coil, and
releasing the magnetic flux to the magnetic disk. The auxiliary magnetic
pole layer has a function of circulating, via the magnetic disk, the
magnetic flux released from the magnetic pole layer. The reproducing
device is, e.g., a magneto-resistance effect device (hereinafter referred
to as an MR device). The magnetic head 103 is mounted on the head gimbals
assembly 108, and arranged to face the magnetic disk medium 1.

[0020] Various magnetic recording media (to be described below) can be
used as the magnetic disk medium 1. An end portion of the head gimbal
assembly 108, on which the magnetic head 103 is not mounted, is fixed to
a distal-end of the carriage arm 106. The carriage arm 106 can be swung
by the VCM using the shaft 110 as the rotation axis. The swinging of the
carriage arm 106 enables the magnetic head 103 to scan on the magnetic
disk medium 1 substantially radially. The magnetic head 103 is positioned
on a desired recording track of the magnetic disk medium 1. Consequently,
the magnetic head 103 can write information to the recording bits
arranged on the recording tracks of the magnetic disk medium 1, or read
information from the magnetic disk medium 1. The head amplifier 107 has a
function of feeding, based on a recoding signal 113, electric current to
the recording device mounted on the magnetic head 103 to record
information on the magnetic disk medium 1, or convert into a reproduction
signal 114 magnetized information on the magnetic disk medium 1, which is
detected by the reproducing device of the magnetic head 103.

[0021] The circuit board 120 includes a read channel 116, a
micro-processing unit (MPU) 115, an SPM driver 111, a VCM driver 112, a
disk controller 117, and the like. The read channel 116 has a function of
decoding the reproducing signals (i.e., servo signals or data signals)
114 supplied from the head amplifier 107 to convert the reproducing
signals into digital information, or to convert information, the
recording of which is instructed by the disk controller 117, into
recording signals 113 for driving the head amplifier 107.

[0022] The MPU 115 drives the VCM driver 112 to perform a positioning
control operation on the magnetic head 103, or drives the SPM driver 111
to perform a rotation control operation on the magnetic disk medium 1,
based on the digital information represented by the servo signal (servo
information) decoded by the read channel 116.

[0023] The disk controller 117 has a function of instructing, according to
a recording/reproducing instruction sent from a host computer 118, the
MPU 115 to position the magnetic head 103 to perform the addressing of
the magnetic head 103 to the magnetic disk medium 1 of the magnetic head
103. In addition, the disk controller 117 has another function of
performing operations of transmitting and receiving digital information
to be recorded and reproduced to and from the read channel 116, and
replying a result to the host computer 118.

[0024]FIG. 7 is a block diagram illustrating an operation performed by
the read channel 116 when reading the servo information recorded on the
magnetic disk medium 1 while the MPU 115 performs a positioning control
operation on the magnetic head 103 in the magnetic disk apparatus having
a magnetic disk medium 1 according to the present embodiment. FIG. 8 is a
timing chart illustrating the operation of the read channel 116.

[0025] An example of the configuration of a servo pattern is such that a
preamble region for clock synchronization is followed by a servo address
mark (SAM) providing a reference timing for reproducing a servo signal,
that the SAM is followed by an address pattern representing a sector
number and a track number, and that additionally, the address pattern is
followed by a burst pattern for detecting the position of the magnetic
head.

[0026] That is, the magnetic disk medium 1 rotates at a constant angular
velocity. Accordingly, servo pattern reproduction signals (a) are
obtained from the head amplifier 107 at constant time intervals. The
servo pattern reproduction signal (a) is subjected to the cutoff of
high-frequency noise components by a low-pass filter 122 in the read
channel 16. Then, the resultant signal undergoes analog-to-digital (A/D)
conversion performed by an A/D converter 123. The gain of a variable gain
amplifier 121 is adjusted, based on digitized amplitude information, by a
gain controller 125 to obtain an optimal amplitude.

[0027] A pattern having a constant cycle is written to an introduction
portion of the servo pattern as a synchronization signal generation
portion 21. A servo gate signal (b) is preliminarily asserted to obtain a
wavenumber sufficient to allow an output of a phase-locked loop (PLL)
circuit 124 to converge.

[0028] When the servo gate signal (b) is asserted, PLL-technique is
applied to the synchronization signal of the servo pattern reproduction
signal. Then, an analog-to-digital conversion (ADC) clock signal (d)
necessary for sampling an address portion and a fine position detection
portion, which appear in the servo pattern reproduction signal, is
generated from the PLL circuit 124.

[0029] At a termination-end of the synchronization signal generation
portion 21 of the servo pattern, a servo sync mark pattern indicating the
start-point of the servo information is written in bits each having a
constant length or in specific code pattern bits. When the servo sync
mark pattern is detected, a synchronization pattern detection signal (c)
is asserted.

[0030] When a synchronization signal detector 126 detects that the
synchronization pattern detection signal (c) is asserted, the
synchronization signal detector 126 sends an address detection gate
signal (e) to an address demodulator 127. Thus, the demodulation of the
address portion to be next produced is performed.

[0031] Upon completion of demodulation of the address portion having a
default length, an address demodulation value (g) is definitely
determined and recorded in a register 129 as digital information. Then, a
burst gate signal (f) is asserted. Thus, the demodulation of the fine
position detection portion is started by a fine position demodulator 128.

[0032] Upon completion of demodulation of the fine position detection
portion having the default length, a fine position demodulation value (h)
is definitely determined and recorded in the register 129 as digital
information.

[0033] With the above operations, the MPU 115 reads the address
demodulation value (g) and the fine position demodulation value (h)
stored in the register 129. Then, the MPU 115 performs calculation
necessary for the positioning control operation on the magnetic head 103.
Thus, the MPU 115 drives the VCM driver 112.

Description of Points (1) and (2) of Embodiment

[0034] According to the present embodiment, an inner end portion of the
servo frame 2k and an outer end of the servo frame 2k+1, which touch a
radius r, are set to overlap with each other by a constant width or1.
Thus, it is implemented to continuously detect a radius. Accordingly, (1)
in a hard disk drive (HDD) apparatus using a bit-patterned medium, each
initial servo frame on the medium is radially segmented into servo frames
at a cycle W. As is seen from FIG. 1, 2 N initial servo frames are
arranged along one circumference of a circle. In addition, (2) as
illustrated in FIG. 1, the number of servo frames, each of which touches
the circumference of a circle having a radius r2 and corresponds to an
associated one of the initial servo frames, is N. On the other hand, the
number of servo frames, which touch the circumference of a circle having
a radius r1 and respectively correspond to the initial servo frames, is 2
N. As illustrated in FIG. 3 enlargedly showing the vicinity of the servo
frames 2k and (2k+1) that touch the circumference of a circle having a
radius r1, the servo frames 2k and (2k+1) are set to radially overlap
with each other by a constant width or1 in the vicinity of a radial
position corresponding to the radius r1.

Description of Point (3) of Embodiment

[0035] In addition, from the viewpoint of recording data on the medium
without waste areas, the gap portion between the servo frames obtained by
segmentation is used as a data area. That is, as illustrated in FIG. 2,
data is recorded on the servo frame gap portion extending from an inner
end r1-or1/2 of the servo frame 2k touching the circumference of a circle
having the radius r1 to an outer end r1-or1/2-W+L, of the next servo
frame arranged at an inner side in the same radial direction. Thus, when
data is recorded on the vicinity of the inner end r1-or1/2 of the servo
frame 2k touching the circumference of the circle having the radius r1,
the reproduction information of the servo frame 2k cannot be used.
Accordingly, it is necessary to perform accurate positioning based on
position information corresponding to (2k+1) servo frames. However, in an
MR-type magnetic recording/reproducing head, a reproducing device and a
recording device are installed by being spaced at a distance in the
direction of a straight-line. If a skew angle is OMR, as illustrated in
FIG. 3, a constant radial MR offset amount therebetween is
ΔθMRr1. To achieve the above accurate-positioning at the
recording of data, it is necessary for recording data at a radial
position r1-or1/2 along the servo frame 2k that the servo frame 2k+1
should extend to at least a radial position r1-or1/2+ΔθMRr1.
The reason is that while a servo frame extending outwardly radially to a
radial position r1+or1/2 is read, data is written to a data track at the
inner side in the radial direction of an adjacent servo frame.

[0036] Thus, according to the present embodiment, the overlapping width
ort, in which the servo frames 2k and 2k+1 overlap with each other, at
the radial position r1 is set as follows: or1≧ΔθMRr1.
Accordingly, the point (3) is that the overlapping width on between the
servo frames at the radial position r1 illustrated in FIG. 4 is larger
than the MR offset amount ΔθMRr1 at the radial position r1.

[0037] Because the skew angle ΔθMR depends upon the absolute
amount of the radius r1, the MR offset amount ΔθMRr1 changes.
The overlapping width or1 can change depending upon the radius.

Description of Point (4) of Embodiment

[0038] A radial continuous length L of the servo frame illustrated in FIG.
2 is determined, based on a condition that no magnetic walls are
generated in the servo pattern configuring the servo frame. FIG. 4
illustrates results of measurement of magnetic-wall occurrence
probability versus radial mark length x in a case where magnetic walls
were generated at an accelerated rate in a magnetic recording medium
having a servo pattern intentionally changed in radial mark length x by
performing a high-temperature environment test within the magnetic
recording apparatus. The magnetic-wall occurrence probability versus the
mark length x depends upon the detailed type of a magnetic film, the
circumferentially maximum width of the servo pattern, or the like.
However, it was found that in this medium, no reverse magnetic domains
were generated when the mark length was equal to or less than, e.g., 50
um. Accordingly, (4) the radial length L of a servo frame having a
certain radius is set such that L=W/2+or1<x_sdomain, based on
comparison with the mark length x_sdomain at which the magnetic film of
the servo pattern configuring the servo frame can maintain the single
domains without generating magnetic walls, the cycle W described in the
point (1), and the overlapping width or1 described in the point (3).
Thus, the magnetic recording medium is designed so that the mark length L
is 50 um.

Description of Point (5) of Embodiment

[0039] When the magnetic recording of data on the gap between the servo
frames is implemented as illustrated in FIG. 3, the recording/reproducing
of data and the reproducing of servo signals cannot simultaneously be
performed at the same circumferential positions, because of restrictions
due to the processing performance of the sector servo system or the
crosstalk of a magnetic recording field onto the reproducing device.
Therefore, even if the positioning of the head to a radial position r1 at
which 2 N servo frames appear at the associated positions arranged in the
circumferential direction is performed, position detection can be
performed only on N servo frames when the recording or reproducing is
performed. Meanwhile, during a seek operation, it is difficult to predict
in advance how the head passes through the gap portion between the servo
frames k and k+1. Both the servo frames k and k+1 may not be randomly
detected at each radius at which the seek operation is being performed.
However, the magnetic disk apparatus is not subjected to the restriction
that the position detection is performed only on N servo frames, though
the apparatus undergoes such a restriction during the
recording/reproducing of data. Accordingly, the influence of the
difficulty of the prediction can be alleviated by performing the
detection on all of 2 N servo frames.

[0040] FIG. 5 illustrates the timings of detection of a head orbit and
servo frames in a case where at moment t1, a recording/reproducing
operation is ended, and a seek operation is started, and where at moment
t2, the end of the seek operation is determined. Until moment t1, the
servo patterns are reproduced at a frequency of N per circumference. When
a seek process is started at moment t1, the frequency of detection of
servo patterns increases to 2 N per circumference. Consequently, the risk
of failure of position detection, which would be caused due to the
passing of the head through the gap while the seek process is performed,
is reduced. Even when no servo gate signal is generated due to a method
illustrated in FIG. 8, the frequency of the detection can be increased by
predicting the timing of opening the gate.

[0041] Then, at moment t2, it is determined whether an output of the PLL
circuit is converged. Thus, the frequency of detection of servo patterns
is reduced back to N. Consequently, the apparatus is brought into a state
in which the recording and reproducing of data can be performed.
Accordingly, in the present embodiment, (5) the detection of servo
patterns is performed at the frequency high than 2 N per circumference.

Description of Advantages of Points (1) and (2) of Embodiment

[0042] Each servo frame is radially segmented. Thus, the generation of
magnetic walls in the servo frame, and the erroneous reproduction of
servo patterns thereon are prevented. The detection of the position of a
normal servo pattern is implemented. In addition, the continuous
detection of a radial position is implemented even in the vicinity of
each segmented portion of the servo frame.

Description of Advantages of Point (3) of Embodiment

[0043] The overlapping width between the segmented portions of the servo
frames is set to be equal to or larger than the amount of the MR offset.
Thus, the recording/reproducing of data to and from the gap portion
between the servo frames are enabled. Efficiency of formatting can be
enhanced. Data can be recorded without waste.

Description of Advantages of Point (4) of Embodiment

[0044] The radius length of the servo frames at practical segmentation is
designed from the relation between the magnetic-wall occurrence
probability and

Description of Advantages of Point (5) of Embodiment

[0045] The frequency of detection of servo patterns at the seek operation
is set at 2 N per circumference. Thus, the failure of the position
detection due to the random passing of the head through the gap between
the servo frames is restricted from occurring. A normal seek operation is
enabled.

[0046] The related art doesn't disclose the configuration in which the
servo frames radially overlap with each other at a certain radius and in
which 2 N servo frames touch the circumference of a circle having the
radius. The related art discloses neither the requirement for determining
the radial length L of the servo frame for preventing the generation of
magnetic walls in the servo patterns configuring the servo frames nor the
control operation performed during the seek operation.

[0047] The risk of occurrence of the erroneous reading of the servo
patterns configuring the servo frames due to the generation of the
magnetic walls in the servo patterns can be removed, as compared with the
related art. The radial width of a non-data part in the radial
discontinuous portion can be minimized, i.e., confined to a level
comparable to the MR offset amount.

[0048] In the bit patterned medium, servo information is degraded due to
the generation of the reverse magnetic domain. The arrangement of servo
frame segments on the medium, and a servo-frame control method for
suppressing this degradation and assuring the formatting efficiency
maximally have been described.

[0049] That is, the embodiment has the following characteristics in
magnetic recording using the bit-patterned medium. [0050] (1) The
embodiment has servo frames formed so that the frequency of the servo
frames is 2 N per circumference of the circle of the medium, that the
segmented servo frames are radially discontinuous, and that the servo
frame and a space-area, on which no servo frames exist, are alternately
radially arranged at a cycle W. [0051] (2) The number of servo frames
arranged to touch the circumference of a circle having a certain radius r
is N per circumference or 2 N per circumference. [0052] (3) If the number
of servo frames arranged to touch the circumference of a circle having a
certain radius r1 is 2 N per circumference, the radial overlapping-width
on between the adjacent ones of 2 N servo frames arranged at a frequency
of 2 N per circumference to touch the circumference of the circle having
the radius r1, which has been described in (2), is set to be equal to or
larger than the read/write offset amount ΔMRr1 of the
magnetic-head, which corresponds to the radius r1. [0053] (4) The radius
length L of the servo frame corresponding to a certain radius is set such
that L=W/2+or1<x_sdomain, based on comparison with the mark length
x_sdomain at which the magnetic film of the servo pattern configuring the
servo frame can maintain the single domains without generating magnetic
walls, the cycle W described in the point (1), and the overlapping width
on described in the point (3). [0054] (5) The detection of servo patterns
is performed at a frequency equal to or higher than 2 N per circumference
during the seek operation.

[0055] While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to limit
the scope of the inventions. Indeed, the novel methods and systems
described herein may be embodied in a variety of other forms;
furthermore, various omissions, substitutions and changes in the form of
the methods and systems described herein may be made without departing
from the spirit of the inventions. The accompanying claims and their
equivalents are intended to cover such forms or modifications as would
fall within the scope and spirit of the inventions.